1. Field of the Invention
This invention relates to a method for tone reproduction which effects reproduction of an image by combination of chromatic inks of cyan, magenta and yellow, together with an achromatic ink of black. More particularly, this invention relates to a method for tone reproduction which, on being used for a color image forming system, such as an ink jet recording system, enables a color image to be reproduced with a tone range amply wide.
2. Description of the Prior Art
Recently, the color image forming system which, in accordance with input color image data, forms on a recording paper a color image has been disseminating in various fields. Among other versions of the color image forming system, the ink jet recording system pre-eminently excels in the ability of tone reproduction and, therefore, is finding utility mainly in the field of activities devoted to the design of apparels and interior articles and to the processing of pertinent images, namely the field which demands high level of tone reproduction.
The ink jet recording system is designed to effect a noncontacting formation of a color image by causing inks of three colors of cyan (C), magenta (M) and yellow (Y), together with an ink of black (K) optionally, to adhere to a recording paper. In the formation of the color image, a certain color is formed by a specific combination of the inks to be adhered with the technique of the subtractive mixture as shown in FIG. 1A. In general, the image with intermediate levels of tones is reproduced with modulation of a dot density in the ink jet recording system because it is difficult to vary dot diameter.
Various inventions have been proposed for formation of a color image. For example, U.S. Pat. No. 4,673,951 discloses an ink jet recording machine which forms a multi-tone color image using four color inks of C, M, Y and K. Japanese Patent Publication SHO 62(1987)-57144 also discloses an ink jet recording machine which reproduces a color image with inks of C, M, Y and K. The machines in both publications varies a dot density of a picture element for intermediate levels of tones. A picture element to be reproduced with all of three inks of C, M and Y has a black component to be represented by overlapping the three color inks each other. Such black component can be reproduced by only the black ink.
Conventionally, two methods of formation of a color image by the aforementioned four color inks are known. On of the methods uses, for reproducing a picture element of a certain color, the three color inks of C, M and Y which have the respective amounts each subtracted by an equal amount corresponding to a portion of the black component of the picture element as well as the black ink which has the amount corresponding to the portion of the black component as shown in FIG. 1B and 1C. Such a treatment of subtracting the equal amount corresponding to the black component from the three color inks of C, M and Y and adding the black ink is referred to as "under color removal (hereinafter referred to as, "UCR" for short)". While the other one of methods uses, for reproducing a picture element of a certain color, the two color inks selected from the three color inks of C, M and Y as well as the black ink. In this method, the two color inks selected from C, M and Y have the respective amounts each subtracted by an equal amount corresponding to the whole black component which coincides the amount of the excepted ink of C, M or Y, while the black ink has the amount corresponding to the whole black .component as shown in FIG. 1D. Hereinafter, the act of the former in which the four color inks is used is called "a partial UCR", while that of the latter in which the three color inks is used is called "a complete UCR".
When the partial UCR is used for formation of a color image, since the four color inks are always used, the respective inks areas often adhered to positions deviating from the proper positions, hence inducing misregistration. The misregistration conspicuously occurs particularly when the achromatic color is reproduced in the region of low levels of tones from the viewpoint of visual sense.
While, when the complete UCR is used, the possibility of the misregistration is completely eliminated because of the reduced number of the inks, however, the sufficient density of the complete black is not obtained. A black ink to be used in an ink jet recording system, must have a lower density to eliminate a possibility of clogging a nozzle through which the ink is spouted (as illustrated in FIG. 2 showing the relation between the density of an original image and that of a printed image by the combination of color inks). Accordingly, the complete black is not sufficiently reproduced by the complete UCR which uses only the black ink to reproduce the complete black.